Integrated circuit memories include a large number of memory cells which are usually set forth in an array. The memory cells may be volatile or non-volatile. If they are volatile, they may be static RAM cells or dynamic RAM cells. There may be one large array, or a given memory chip may have several sub-arrays which may be arranged in blocks. Typically, the memory device comprises a large number of bit lines extending in one direction. The bit lines may be paired or non paired. Where they are paired, they are often referred to as complementary bit lines, or paired bit lines. Complementary bit lines are used for both static RAM and dynamic RAM applications. The bit lines generally extend in a first direction, and a plurality of word lines extend in a second direction which is perpendicular to the first direction. Typically, a memory cell is located at or near the crossing of a bit line with a word line.
Often, a memory array is divided into subarrays, and each subarray may further be divided into blocks. Each subarray will often have its own "peripheral circuitry" such as decoders.
The memory array or subarray is usually said to be organized into rows and columns. Generally, a row indicates the memory cells located along (coupled to) a word line. A column is therefore ordinarily perpendicular to a row and generally indicates a collection of memory cells along (coupled to) a bit line or a bit line pair. Generally, each column is connected to a respective sense amplifier. One job of the sense amplifier is to sense the effect that the memory cell has on the bit line(s) and to amplify that signal for outputting in a read operation. Conversely, the sense amplifier may also drive or control the bit line(s) when the memory is writing data into a memory cell.
CMOS technology is prevalent today. FIG. 1 illustrates a prior art configuration and shows a sense amplifier 10 using CMOS technology. The operation and configuration of such a sense amplifier 10 is well known and will not be explained except briefly herein. Amplifier 10 contains P channel transistors 12 and 14 having source electrodes commonly coupled to a line 16 which carries from time to time a signal LP also called LATCHP. The sense amplifier also comprises a pair of N channel transistors 18, 20 having source electrodes commonly coupled to a line 22 which sometimes carries a signal which may be called LN or LATCHN. A pair of internal nodes A, B are connected to gate electrodes. In particular, node A is coupled to the gate electrodes of transistors 12 and 18, while node B is coupled to the gate electrodes of transistors 14 and 20. These transistors form a latch. A first bit line BL1 on the left side of sense amplifier 10 is coupled to node B which is also coupled between the drain electrode of P channel transistor 12 and the drain electrode of N channel transistor 18. Likewise, a complementary bit line BL1 BAR is connected to node A which is also coupled between the drain electrode of P channel transistor 14 and N channel transistor 20. Transistors 12 and 14 are referred to as "pull-up" transistors whereas transistors 18 and 20 are referred to as "pull-down" transistors. When a transistor 24 is turned on, it couples VCC through its source-drain path to line 16, thereby providing the LATCHP signal. Sense amplifier 10 forms a flip-flop so that either transistor 12 or 14 but not both will be turned on and will pull the voltage at its corresponding node toward VCC. At or near the same time, one of the transistors 18 or 20 will pull down the voltage at the other node toward VSS which will be connected to line 22 via a transistor 26 being turned on. In this way, one of the two nodes is pulled high and the other is pulled low, and the sense amplifier latches into a stable state.
In any large memory, such as a 16 megabit DRAM, there will be thousands of columns and thousands of rows. This is represented in FIG. 1 which shows a second sense amplifier 30 connected to corresponding bit line pair BL2 and BL2 BAR, and an N-th sense amplifier 32 coupled to bit lines BLN and BLN BAR. It should be appreciated that N may be on the order of 1000 or more. The LATCHP signal is applied to all N of these sense amplifiers via line 16, and the LATCHN signal is applied to them via the line 22. It will be seen in FIG. 1 that a plurality of resistances 34 are illustrated. These are not discrete resistance devices but rather indicate the parasitic resistance of the lines 16 and 22, which, even though they are formed of conductive materials such as metal or the like, nevertheless over great distances will have some resistance value. Over each resistance, there will be a voltage drop from the voltage applied via transistor 24 or 26, as the case may be. Accordingly, the voltage that eventually reaches sense amplifier 32 may be appreciably diminished from VCC or VSS, and that sense amplifier will work inefficiently or slowly. It will also be appreciated that because of this problem, sense amplifier 10 does not activate at the same time as sense amplifier 32 and the resulting skew prolongs access time. Additionally, some prior art designs can be unstable if the selected sense amplifier is connected to the data line (the bit lines) too early.
Thus, as power supply (VSS) line 22 is trying to pull down to 0v, transistors start to turn on in the sense amplifiers. A current flows to the right on line 22, and there exists a voltage drop due to the resistance of line 22. Practical limitations prevent the solution of greatly widening line 22 to reduce its resistance--the chip area is jealously allocated. Hence, in the illustrated architecture, the right-most sense amplifier 10 turns on first, and sense amplifier 32 will turn on thereafter.
Generally, one desires to pull down line 22 at a controlled rate. The far end (most remote from transistor 26) of line 22 will drop in voltage slower than the near end. This slows the memory, which is undesirable, but if circuitry drove the near end too fast, then the corresponding near sense amplifiers would become unreliable.
Another problem occurs when the near sense amplifiers latch logic "1's" and the far amplifier latches a logic "0." There is a pattern sensitivity because when the bit lines are precharged to 1/2VCC, the memory cell moves only one of the bit lines lower or higher. Sensing a "1" occurs before sensing a "0" because LN needs to drop only 1 Vt below a voltage level corresponding to a "1." However, to sense a "0" LN must be 1Vt below 1/2VCC, and this occurs later. Large current flows when "1's" are read. Because of the large currents, the decline in voltage at the far end of line 22 slows to an uncontrolled rate. This effectively can add 7 nsec. to the sensing process--a substantial and undesirable increase.
One approach that has been proposed for addressing this problem is depicted in FIG. 2. It shows the same sense amplifiers 10, 30, and 32, and has the same signals LATCHP and LATCHN provided by transistors 24 and 26 respectively. However, further N channel transistors have been added beneath the sense amplifiers and a modification has been made so that LATCHN is carried by two distinct lines. One of these lines 40 is constructed relatively wide to carry most of the current, and the other line 42 is relatively narrower because it will carry current for just a single selected sense amplifier. Line 40 is coupled to the sources of a plurality of transistors 44, each sense amplifier having a respective transistor 44. Each transistor 44 has its gate electrode coupled to VCC and is therefore generally on. Transistors 44 are relatively small in size so that they do not carry much current to any single sense amplifier.
The narrower line or rail 42 is coupled to the several sense amplifiers by source-drain paths of respective transistors 46, which are shown also as N channel transistors. Transistors 46 are decoded and this is indicated by a low to high transition signal YR 47 shown beside the gate electrode of transistor 46 for sense amplifier 32 at the far left side of FIG. 2. The other transistors 46 also are coupled to their YR signals, which are shown illustratively at 0v, i.e. they are not selected columns. Thus, the column which has been selected decodes its YR signal to apply to the gate electrode of the transistor 46 thereby to couple the narrow LATCHN line 42 to the sense amplifier. See also Okamura et al., "Decoded-Source Sense Amplifier for High-Density DRAMs", IEEE J. Solid State Circuits, Vol. 25, No. 1 (February 1990), pp 18-23. This solution does reduce the sensing skew since the current flowing through line 40 is reduced due to the weak transistors 44 in the current path. Therefore, the voltage drop down line 40 is reduced. Disadvantages of this approach are that the large transistors 46 must be added and that the capacitive loading on the column select signal YR is increased since it must drive the gates of transistors 46. There is poor control over the current draw and extra loading to the YR line because it is driving an extra transistor per sense amplifier.
Chin et al., "An Experimental 16-Mbit DRAM with Reduced Peak-Current Noise," IEEE J. Solid State Circuits, vol. 24, no. 5 (October 1989) at p. 1191 et seq. and particularly in FIG. 4 adds both p-channel and n-channel transistors between sense amplifiers and power supply lines. However, FIG. 4(a) does not use one connection to the VCC line and one connection to the VSS line per sense amplifler. It does not drive these power lines from one end only, but rather from several connections that are distributed in the array. Signals SAP and SAN bar may correspond to LP and LN respectively. Thus, in the architecture disclosed in that article, there are several sense amplifiers connected together, and there will still be some pattern sensitivity.
FIG. 4(b), however, in view of the caption for the figure and the accompanying text, could mean that each sense amplifier has a respective pair of local P channel and N channel drive transistors. Nevertheless, FIG. 4(b) shows that all of the P channel transistors (for multiple sense amplifiers) are connected to a single node SAP. All of the N channel transistors are connected to a single node SAN bar.
Chin et al. U.S. Pat. No. 4,948,993 similarly shows such common nodes S and S bar in FIG. 2, but FIG. 3 thereof avoids such common nodes.
FIG. 3 illustrates further transistors that have been used in the past or are used in current generations (16 Meg) of very large capacity integrated circuit memories. Thus, sense amplifier 10 is coupled between LATCHP and LATCHN signals which are generally VCC and VSS. The data signals coming from the data lines are illustrated as D and its complement D BAR. However, the source-drain paths of further pass transistors 48 and 50 couple the data signal D to the sense amplifier, and an identical arrangement is provided for the complementary data signal. Transistor 48 is responsively coupled to a global column select signal Y which is applied to the gate electrode thereof. Most 16 meg DRAMs have such global Y select signals. Additionally, very large scale memories, as mentioned above, include one or more blocks, and transistors 50 are illustrated to show a block enable signal controlling the operation of transistor 50. In this, the complementary data signals must be high, near VCC, during reading and the pass transistors must be small compared to the LATCH transistors to avoid instability.
FIG. 4 shows some of the problems of circuitry styled along the lines of FIG. 3. In FIG. 4, three identically constructed sense amplifiers 10 are illustrated, and for ease of reference they will be referred to in this drawing as 10A, 10B and 10C. For ease of illustration, the LATCHP line 16 and LATCHN line 22 are not shown. The column select signal Y is a global select signal and is therefore shown as having a voltage of VCC. The block enable signals are separately provided so that the pass transistors 50A for sense amplifier 10A have their gate electrodes coupled to a line 52A which carries a first block select signal. Similarly, pass transistors 50B for sense amplifier 10B have their gate electrodes coupled to a second block select line 52B which carries a second block select signal, and a block select signal line 52C similarly corresponds to sense amplifier 10C.
Sense amplifier 10A is illustratively in an inactive block, for sake of explanation. "Inactive" means herein that LN and LP are at 1/2VCC and the latch is inactive. Its block has not been enabled so the block enable signal on line 52A is low at zero volts. Thus, pass transistors 50A are off, and regardless of whether transistors 48A may be on, sense amplifier 10A is generally isolated from data. However, to prevent it from latching, LATCHP and LATCHN must both be kept at 1/2VCC, and the data write signal DW and its complement DWB are held at VCC.
Sense amplifier 10B is in an active block with LN at 0v and LP at VCC. However, with the block select at 0v, no write will occur even with the global column select Y at VCC.
Since amplifier 10C is in an active block and a write operation is to occur through amplifier 10C. The block select signal on line 52C is high, at VCC. The column select signal Y is high. With the data signal DW high at VCC and its complement DWB low at zero volts (or vice-versa if different data is being written), the pass transistors 48C, 50C on at least one side of sense amplifier 10C will turn on, and the sense amplifier will latch the data state and drive the bit lines so that the data state will be written into the memory cell or cells along the column (usually at the active word line).
An object of the present invention is to improve the sense amplifiers to overcome or reduce the aforementioned problems.